Sunscreens are chemical ingredients which are usually applied topically to the skin and hair in order to protect them from the damaging effects of the sun's radiation, especially against (UV) ultraviolet radiation.
There is evidence that prolonged sunlight exposure results in increasing incidence of skin carcinogenesis, pigmentation, anomalies and precancerous lesions such as actinic keratosis, melanoma and nonmelanoma skin cancers, as well as accelerated skin aging and undesirable changes in hair quality.
Sunscreens are usually classified in two major categories. The first includes chemical sunscreens which absorb sun radiation and therefore reduce the amount of UV radiation reaching the skin. Chemical sunscreens can be subdivided into eight derivative families: para amino benzoates, salicylates, cinnamates, benzophenones, anthranilates, dibenzoylmethanes, camphores and miscellaneous chemicals. The second category includes physical sunscreens which reflect, scatter or physically block the UV light reaching the skin surface. Physical sunscreens are mainly metal oxides such as titanium dioxide, zinc oxide and also red petrolatum.
Ultimate sunscreens should be chemically inert, highly photostable and neither sensitizing nor photosensitizing. Yet, according to more than 200 published reports relating to all major chemical sunscreen families, sunscreen agents induce both contact and photocontact dermatitis (Dromgoole and Maibach, In Sunscreens, eds. N. J. Lowe, N. A. Saath; Marcel Dekker: N.Y., 1990, Chapter 20). In other publications it was reported that commonly used sunscreen compounds undergo photodecomposition reactions (Roscher et al. J. Photochem. Photobiol. A: Chem, 80: 417-421, 1994). Moreover, although more people are using sunscreens, the rate of skin cancer around the world is rising. A possible explanation is that sunscreens might encourage, rather than prevent, sun related cancers. Indeed, it was found (Knowland et al. FEBS Letters, 324: 309-313, 1993) that the commercial sunscreen ingredient Padimate-O is mutagenic in sunlight. The sunlight-excited Padimate-O is thought to produce free radicals which directly attack the DNA.
Physical sunscreens, especially titania, such as disclosed in Chemical Abstracts 124:153660 although considered to be "relatively safe", are known to be photosensitizers capable of rupturing covalent bonds (U. Stafford, K. A. Gray and P. V. Kamat, Heterogeneous Chem Rev, 3, 77-104, 1996).
There is, therefore, a great need for isolating chemical and physical sunscreen agents from the body while retaining both the sunscreen activity and the compatibility with cosmetic preparations.
U.S. Pat. No. 5,223,250 and WO Patent 95/28912 describe the entrapments of sunscreen ingredients in organic polymers. However, organic polymers suffer a major drawback in that they do not provide photostability, a most important requirement of a sunscreen matrix. The photodegradation, photooxidation and photoreactivity of organic polymers are well documented (summarized in J. F. Rabeck, Photodegradation of polymers; Springer: Berlin, 1996). Also, plastic carriers, as in the above mentioned patents, are unable to tightly entrap dopants. Small molecules are known to diffuse in and out of these polymers, making the isolation from body tissues incomplete.
The present invention relates to sunscreen-doped sol-gel materials, such as chemical and physical sunscreens that are doped in a transparent sol-gel matrix, for the purpose of protecting body tissues (the term "body tissues" in the present invention refers to body tissues that are exposed to sun light such as hair, skin and nails) and other surfaces (the term "surfaces" in the present invention refers to surfaces that are exposed to sun light and may be damaged from the exposure to UV radiation) from ultra violet (UV) radiation, to a method for the preparation of these sunscreen-doped sol-gel materials and to a method for the protection of body tissues and other surfaces exposed to sunlight, from UV radiation, using the said sunscreens.
In the present invention the term "sunscreen-doped sol-gel" refers to sunscreen molecules or polymers (capable of absorbing or deflecting UV radiation) when they are doped in sol-gel matrix.
The term "chemical sunscreen" in the present invention refers to chemical ingredients which absorb sun radiation and therefore reduce the amount of UV radiation reaching the skin or other surfaces. The term "physical sunscreens" in the present invention refers to chemical ingredients which reflect, scatter or physically block the UV light reaching the skin or other surfaces.
U.S. Pat. Nos. 5,292,801 and 5,300,564 (inventors: Avnir et al) describe the preparation of the inorganic porous ceramic glass-like sol-gel matrix obtained by hydrolysis and condensation-polymerization of metal and semi metal alkoxides resulting in oxide-type materials such as SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, ZnO and similar materials, as well as in their mixtures and their covalent bound organic derivatives. Due to the relatively low temperature needed for the preparation of sol-gel matrices, organic molecules can be doped into the matrix.
Sol-gel matrices doped with organic molecules are disclosed for example in EP 0281034 in which the organic molecules are perfume molecules and in Patent Abstracts of Japan Publication No. 08099838 in which the organic molecules are color molecules.
The sol-gel matrix of the present invention is thermally and photochemically stable, as opposed to the above mentioned (U.S. Pat. No. 5,223,250 and WO 95/28912) plastic carriers, and can easily withstand normal duration of exposure to sunlight. Also, as opposed to the plastic carriers of the above patents, no leaching of the trapped sunscreen agents occurs from the sol-gel matrix, so that there is no direct contact of the sunscreen molecules with the body tissues.
The present invention relates to a simple method for preparing a matrix entrapping any sunscreen molecule or particle, providing, for the first time, a stable, safe and environment friendly sunscreen.